Variable gain amplifier (VGA) circuits are useful in many applications. One particular application is in rechannel cores for disk drives. As is known in the art, such rechannel cores may be embodied as integrated circuits for use in a disk drive. As is known in the art, from the reading head of the disk drive at the edge of the reader arm, an analog-to-analog fixed gain amplifier resides at the disk drive read/write arm. After the read amplifier, a second amplifier is configured to give a constant output signal level. In order for this second amplifier to provide a constant output, it must have a variable gain because the magnitude of the input will vary according to varying operating parameters, such as the head of the disk drive flying closer or further from the magnetic surface, variations in preamplifier gain, or other varying operating parameters.
D. R. Welland et al., "A Digital Read/Write Channel with EEPR4 Detection," ISSCC Dig. Tech. Papers, February 1994, pp. 276-277, discusses one rechannel circuit design. The Welland paper, FIG. 2 in particular, shows a structure with NMOS inputs, current source loads, source followers, dual single ended feedback, and NMOS GDS sets input transconductance. However, the circuit shown has a current output, an NMOS feedback, and a static control voltage (VCON). This circuit is therefore not suitable for some applications, particularly because common mode input signals modulate the conductance of the gain setting transistor. The present invention instead ties gain control to the VGS of the input NMOS the current sources relative to the CMOS feedback supply voltage regulator, giving superior performance.
G. Vishakhadatta et al., "An EPR4 Read/Write Channel with Digital Timing Recovery," IEEE J Solid-State Circuits, Vol. 33, No. 11 November 1998 pp. 1851-1857 discusses (see FIG. 4, p. 1154) a circuit with an input stage that has some similarities to the present invention, but with voltage outputs from the source follower gates, and replacing the NMOS gain control of Welland with a resistor for better linearity. However, the resistor has a fixed conductance, requiring the gain to be set in the feedback devices. In that VGA, I1 & I2, the tail current is used to set the gain, but in an inverse relationship to the present invention, because the current through the input devices sets the feedback conductance, rather than the input conductance.
F. Krummenacher, "A 4 MHz Continuous-time filter with on-chip automatic tuning," IEEE Journal of Solid-State Circuits, vol. SC-23, pp. 750-758, June 1988, discusses using two NMOS in triode with gates to inputs to linearize transconductance.
What is needed is a method or apparatus that can provide variable gain amplification with high bandwidth, efficiency, and controllable gain and bandwidth.